1. Field of the Invention
The present invention relates to a display device. In particular, the present invention relates to the structure of a scanning line driver circuit in an active matrix display which includes a light-emitting element and which is manufactured by using a semiconductor device.
A semiconductor device herein described indicates any device which can function by using a semiconductor characteristic.
2. Description of the Related Art
In recent years, demand for thin displays mainly applied to TVs, PC monitors, mobile terminals, or the like has increased rapidly, and further development has been promoted. The thin displays include a liquid crystal display device (LCD) and a display device equipped with a light-emitting element. In particular, an active matrix display using a self-light-emitting element is expected as a next-generation display for its features of high response speed, wide viewing angle, and capability of bending with the use of a flexible substrate or the like, in addition to advantages of a conventional LCD such as thinness, lightness in weight, and high image quality.
In an active matrix display using a light-emitting element, a structure shown in FIG. 16B is given as the most basic pixel structure (see Non-Patent Document 1:M. Mizukami, K. Inukai, H. Yamagata, et al., Society for Information Display '00 Digest, vol. 31, pp. 912-915). As a driving method of the pixel structure, there are a current driving method, by which a driving transistor is driven in a saturation region and emission luminance is controlled by current applied to the light-emitting element, and a voltage driving method, by which the driving transistor is driven in a non-saturation region (a linear region) and light emission is controlled by voltage applied to the light-emitting element.
With the above method of current driving, it is necessary to operate the driving transistor in the saturation region. Ideally, current flowing through the light-emitting element does not depend on drain-source voltage (Vds) but on gate-source voltage (Vgs) of the driving transistor. However, when there is variation of characteristics of the driving transistor in each pixel due to a manufacturing process of the transistor or the like, the amount of current flowing through the light-emitting element is affected. In particular, when effective Vgs (hereinafter, referred to as eVgs) is non-uniform depending on pixels due to the variation of threshold voltage (hereinafter, referred to as Vth) of the transistor, emission luminance differs widely among pixels, which results in display unevenness and deterioration of display quality.
Thus, it is desirable to employ a voltage driving method by which a driving transistor is operated in a linear region less subjected to the variation of Vth. As a gray-scale method of the case of driving voltage, a digital time gray-scale method by which a gray scale is displayed with the continuous change of a light-emitting time. In addition, in the digital time gray-scale method, there are problems such as high speed of a signal line driver circuit, increase of a video signal in division number, and increase in number of transistors of each pixel. As a means for solving these problems, a driving method, by which a scanning line selection period is divided back and forth and a writing period and an erasing period of a signal are provided alternately, has been proposed (see Patent Document 1: Japanese Published Patent Application No. 2005-338777).